In the prior art, regenerative thermal oxidizers are known for oxidizing pollutants, such as hydrocarbon vapors in air, and converting the pollutants into carbon dioxide and water vapor. Typically, a pollutant laden "dirty" gas to be cleaned is directed into a combustion chamber and through a previously heated regenerative heat exchanger. At the same time, a previously combusted hot "clean" gas is directed out of the combustion chamber and into a second heat exchanger. The gas to be cleaned leading into the combustion chamber is heated as it passes through the previously heated heat exchanger, while the gas which has been combusted is passing out through the second heat exchanger, heating the second heat exchanger. In this way, regenerative thermal oxidizers continuously operate to combust or oxidize a gas to be cleaned. By alternating the flow of cool gas to be cleaned through a hot heat exchanger, then moving hot gas from the combustion chamber outwardly through a heat exchanger, each heat exchanger is periodically alternatively heated and cooled.
Known regenerative thermal oxidizers have valving systems which periodically switch the inlet flow of gas to be cleaned between the several heat exchangers, and periodically switch the outlet flow of clean gas between the several heat exchangers. Thus, each heat exchanger is periodically moved from receiving gas to be cleaned, which is heated by the heat exchanger, and then subsequently receives a combusted clean gas which heats the heat exchanger.
A problem exists with the prior art devices in that when a particular heat exchanger is initially switched from receiving a gas to be cleaned to receiving a gas which was cleaned, there is residual dirty gas to be cleaned remaining in the heat exchange structure, which will be exhausted to the environment.
The prior art regenerative thermal oxidizers typically have utilized a plurality of small pieces of ceramic material as heat exchange media. Typically, the heat exchanges for regenerative thermal oxidizers have included one inch ceramic saddle-shaped pieces, or irregular mineral spheroids or gravel. The saddles or spheroids are poured into a regenerator shell and raked to a uniform depth. The individual pieces of the heat exchange media remain in whatever orientation they happen to fall into when the regenerator shell is filled. The resistance to gas flow or pressure drop through the heat exchange media is relatively high and will vary through the heat exchange media, depending upon the random orientation of the media and, to some extent, the degree of contamination. In a typical regenerator having randomly oriented saddle-shaped pieces, the overall pressure drop will be about ten inches of water, or greater.
As mentioned above, problems remain with such heat exchangers in that when a particular heat exchanger is initially switched from receiving a gas to be cleaned to receiving a gas which is cleaned and is to be delivered to an outlet, any residual inlet, or "dirty" gas to be cleaned remaining in the heat exchange medium will be delivered to the outlet as clean gas. When the particular heat exchanger is initially switched into a mode of receiving a clean gas, that clean gas will entrain some dirty gas and move it outwardly to the outlet line. The outlet line is normally released to the atmosphere.
Strict laws prevent the discharge of any pollutants to the atmosphere. Thus, there is a need to eliminate any residual gas to be cleaned remaining in the heat exchanger when it is initially switched to receiving clean gas. Such a need is difficult to achieve with standard regenerative equipment.
On the other hand, the use of the regenerative heat exchangers provides valuable benefits in that it preheats the gas to be cleaned on the way to the combustion chamber. Thus, it is possible to obtain almost complete combustion in a very short period of time. This allows processing of industrial gasses which contain pollutants, such as volatile solvents, in a practical and expedient manner. For that reason, it would not be desirable to eliminate the regenerative function.
One solution to the problem of residual gas is the inclusion of a "purge" system into the regenerative thermal oxidizer. The use of a purge system can be best visualized in a system with at least a third heat exchanger. A first heat exchanger would typically be in an inlet mode receiving a gas to be cleaned, a second heat exchanger is being purged by a clean gas, and a third heat exchanger is in an outlet mode receiving the combusted gas from the combustion chamber. The purge cycle may tap gas from a downstream location on the clean gas and return it through the second heat exchanger and into the combustion chamber. This purge gas drives any residual gas to be cleaned from the heat exchanger and into the combustion chamber where it can be cleaned before being delivered to the atmosphere. Such purge systems have proven effective in reducing the amount of residual gas.
Even so, there may be residual gas left in the regenerative thermal oxidizers on some occasions. Applicant has discovered that in large part, the remaining residual gas may be due to the heat exchange media used in the typical regenerative thermal oxidizers. The use of the saddles or spheroids provides many diverse and partially enclosed spaces to receive the gas; thus, it is quite difficult to thoroughly drive all residual gas to be cleaned from the heat exchange medium.
In addition, since the flow passages vary and have no predictable shape, size or direction, the pressure drop across the heat exchanger may have local variations. The overall pressure drop is typically relatively high. These problems relating to the pressure drop also contribute to residual inlet gas to be cleaned remaining in the heat exchanger.
It is most important to insure that the regenerative thermal oxidizers continue to operate at all times. A primary use of such systems is to process air from paint spray booths to remove volatile solutions or paint vapors from the air prior to discharge to atmosphere. In order to process the maximum amount of air, it is desirable to insure that each heat exchanger is in a inlet mode or an outlet mode for the maximum possible amount of time. Thus, it is desirable to reduce the timing of the purge cycle relative to the inlet and outlet cycles. In regenerative thermal oxidizers the purge cycle typically does not take as long as the inlet or outlet cycles, and thus two of the heat exchangers are more often in an inlet or outlet mode in a standard three heat exchanger regenerative thermal oxidizer. With the prior art heat exchanger media formed of the loose, randomly oriented particles, it was necessary to maintain the purge cycle for an undesirably long period of time. This was due to the fact that the dirty residual air could be found in any of the diverse or partially enclosed spaces defined by the loose heat exchange medium particles, and also due to the problems relating to pressure drop.